IPC-TM-650 EN 2022 试验方法.pdf - 第483页

Step 3 – Calculate the reflection coefficient of the transmis- sion line under test relative to the air line. If the TDR system already provides reflection coefficient values, go directly to Step 4. The reflection coeffi…

directly

to Step 5. The reflection coefficient, ρ

for

the trans-

mission line is given by:

r,0

i,0

Step

5 –

Calculate

the characteristic impedance, Z

C,X

, of

the

transmission line using:

C,x

= Z

std

1 +ρ

1 −ρ

The Z

C,x

, therefore,

corresponds to the average, minimum, or

maximum characteristic impedance value of the transmission

line under test.

5.2.3

Stored Reference Waveform Method

The

stored

reference method requires a fewer number of measurements

than the transfer standard method (see 5.2.1) because the

reference waveform is stored and subsequent waveforms

compared to it. However, it is important in this method that

the amplitude offset of the TDR unit does not drift.

5.2.3.1

Measurement Calibration Procedure

The

instru-

ment setting must be the same for Steps 1 and 2. This pro-

cedure will determine the characteristic impedance of the

transfer standard from which characteristic impedance of the

transmission line under test will be determined (see 5.2.1.2).

Step

1 –

Hold

the probe in air (see Figure 5-11) and measure

the average voltage levels for each of those parts of the TDR

waveform corresponding to the open step, V

open

, and

to the

reference standard, V

std,0

Calculate the amplitude, V

i,0

fthe

incident voltage step using:

i,0

= V

open

− V

std,0

Step

2 –

Record

the voltage, V

check,0

, in

the TDR waveform

corresponding to the precision RF cable.

5.2.3.2

Measurement Process

The

instrument setting

must be the same for Steps 1 and 2. This procedure will

determine the characteristic impedance of the transmission

line under test. This process should be done after the mea-

sure zone has been defined (see 5.1.3).

Step

1 –

Probe

the transmission line (see Figure 5-12) and

measure the mean, minimum, and maximum voltage values,

C,ave

C,min

, and V

C,max

, of

that part of the TDR waveform

corresponding to the measurement zone of the transmission

line.

Step

2 –

Record

the voltage, V

check,1

in the TDR waveform

corresponding to the precision RF cable and compute the fol-

lowing:

diff

check,0

− V

check,1

i,0

If V

diff

greater than 0.002, then the TDR system must be

recalibrated (see 5.2.3.1).

IPC-2257a-5-11

Figure

5-11 Calibration for Stored Reference Method

AIRLINE

PROBE

SPD

TDR

INSTRUMENT

std,0

open

PRECISION

RF CABLE

check,0

f,std

i,std

MEASUREMENT ZONE

for AIR LINE

1,TL

TIME

i,0

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

14 of 23

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Step

3 –

Calculate

the reflection coefficient of the transmis-

sion line under test relative to the air line. If the TDR system

already provides reflection coefficient values, go directly to

Step 4. The reflection coefficient, ρ

tran

for

the transfer stan-

dard is given by:

tran

C,x

− V

std,0

i,0

Step

4 –

Calculate

the impedance, Z

tran

, of

the transmission

line under test using the following formula:

tran

= Z

std

1 +ρ

tran

1 −ρ

tran

5.3

Differential TDR Measurement Procedures

This

section

contains one method for measuring the characteristic

impedance of differential transmission lines. The following cali-

bration and measurement steps should be used when the

device(s) under test are differential (balanced) transmission

lines. This process can be followed manually but, to improve

measurement repeatability and reduce measurement time, an

automated measurement system is recommended. Addition-

ally, the use of a fixture based or robotic probing system

greatly improves the accuracy and repeatability over hand

probe techniques and further reduces the measurement time.

The differential method described herein requires that (1) the

probe act as a transfer standard (see 4.3.3.1), (2) the TDR

system uses a source that delivers a differential signal to the

signal lines of the differential transmission line, and (3) the TDR

samples the reflected signal from both signal lines of the dif-

ferential transmission line simultaneously. In this case, two

TDR waveforms are obtained, one from each of the signal

lines of the differential transmission line, and these TDR wave-

forms are used to determine the odd mode impedance of

each signal line of the differential transmission line. The odd

mode impedance is not the same as the characteristic imped-

ance of a single-ended transmission line. These odd mode

impedances are then used to compute the differential imped-

ance of the differential transmission line. For a discussion of

differential impedance propagation modes refer to IPC stan-

dard 2141.

5.3.1

Instrumentation and Equipment

The

TDR instru-

ment used in this test method provides two opposite-polarity

equal-magnitude signals that are simultaneously applied to

the two signal lines of the differential transmission line under

test. A typical TDR waveform from such an instrument is

shown in Figure 5-13.

Follow the manufacturer’s recommended procedure and

schedule for in-house as well as factory calibrations of the

TDR. Prior to beginning each measurement and no less than

once daily, check the skew between the two output signals of

the differential source of the TDR. This skew should be

checked at the output end of the probe not at the TDR output

or at the end of the connecting cables. The skew should be

adjusted to be a minimum. Check that the amplitude of the

differential signals have equal but opposite polarity and verify

that they are in specification. Use the appropriate probes (see

4.3.3.1).

IPC-2257a-5-12

Figure

5-12 TDR Measurement of Transmission Line

PROBE

SPD

TDR

INSTRUMENT

C,ave

PRECISION

RF CABLE

check

TRANSMISSION LINE UNDER TEST

MEASUREMENT ZONE

for TRANSMISSION

LINE UNDER TEST

f,TL

i,TL

TIME

r,1

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

15 of 23

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5.3.2

Determination of Measurement Zone

The

deter-

mination of the measurement zone is done similarly to that for

single-ended transmission line test methods as discussed in

5.1.3. However, for this differential test method, the TDR

waveform will appear similar to that shown in Figure 5-14.

The instants shown in Figure 5-14, t

i,TS

f,TS

i,TL

, and t

f,TL

are

the initial and final instants of the measurements zones for

the transfer standard and transmission line under test. The

i,TS

f,TS

are

the same as t

i,Ref

and t

f,Ref

described

for single-

ended transmission lines in 5.1.3.

5.3.3

Measurement Calibration Procedure

The

instru-

ment setting must be the same for Steps 1 and 2. This pro-

cedure will determine the characteristic impedance of the

transfer standard from which characteristic impedance of the

transmission line under test will be determined (see 5.3.4).

Step

1 –

Hold

the probe in air and measure the average volt-

age levels corresponding to the high and low states of the two

differential TDR waveforms, which are labeled V

TS,Ch1,1

TS,Ch2,1

, V

open,Ch1

and V

open,Ch2

Figure 5-15. There are a

total of four states, two for each of the differential waveforms.

Calculate the amplitude, V

inc,Ch1

of the incident voltage step

for Channel 1 (Ch1) using:

inc,Ch1

= V

TS,Ch1,1

− V

open,Ch1

and

the amplitude, V

inc,Ch2

of the incident voltage step for

Channel 2 (Ch2) using:

inc,Ch2

= V

TS,Ch2,1

− V

open,Ch2

where:

TS,Ch1,1

the average voltage level of that part of the Ch1

TDR waveform corresponding to the transfer standard,

TS,Ch2,1

the average voltage level of that part of the Ch2

TDR waveform corresponding to the transfer standard,

open,Ch1

the average voltage level of that part of the Ch1

TDR waveform corresponding to the high state of the differen-

tial signal applied to Ch1, and

open,Ch2

the average voltage level of that part of the Ch2

TDR waveform corresponding to the high state of the differen-

tial signal applied to Ch2.

IPC-2257a-5-13

Figure

5-13 Differential TDR Waveform

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Time

Signal (V)

Ch1

Ch2

ransmission Line

Measurement Zone

IPC-TM-650

Number

2.5.5.7

Subject

Characteristic

Impedance of Lines on Printed Boards by TDR

Date

03/04

Revision

Page

16 of 23

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